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Модуль index

Controls how to iterate over tuples in an index. Different index types
support different iterator types. For example, one can start iteration
from a particular value (request key) and then retrieve all tuples where
keys are greater or equal (= GE) to this key.

If iterator type is not supported by the selected index type, iterator
constructor must fail with ER_UNSUPPORTED. To be selectable for primary
key, an index must support at least ITER_EQ and ITER_GE types.

NULL value of request key corresponds to the first or last key in the index,
depending on iteration direction. (first key for GE and GT types, and last
key for LE and LT). Therefore, to iterate over all tuples in an index, one
can use ITER_GE or ITER_LE iteration types with start key equal to NULL.
For ITER_EQ, the key must not be NULL.